This invention relates to Controlled Low-Strength Mixtures (CLSMs), or flowable back-fills. This class of materials has utility as pipe bedding materials where they are used to both protect the pipe from external agents and internal loads. They have also been used as an erosion barrier in embankments and as a mine fill material. CLSMs typically have strengths of less than 2000 psi and, in cases where removal is contemplated, less than 200 psi for ease of removal. The material should be initially in the form of an easily pumpable, self-leveling slurry. Rapid early strength development (approximately 50-70 psi) is a desirable property and is currently not obtainable with commercial products without the penalty of high strength development at later stages. U.S. Pat. No. 5,106,422 discloses Class C Fly ash in a rapid setting flowable backfill composition and method for its use. However, such existing compositions are based upon the use of either Portland cement or Class C fly ash used individually or in combination as the hydraulic cement component of the CLSM system. Typically these cementitious materials are used at less than 5% by weight in the case of Portland cement or as much as 50% in the case of Class C fly ash with the remainder being some form of aggregate, usually fine sand or soil from the spoil with small amounts of additional rock and gravel or Class F fly ash. Cement-based materials can take days to hydrate, cure, and achieve even a modest strength of 50 psi which is typically the minimum strength required for a man to walk upon the surface of the bedding material and represents the minimum safe time before the cover fill may be placed. Class C fly ash based systems may take as long as four hours to hydrate, cure, and achieve this strength. In many cases, locally available Class C fly ash is not desirable for use in these types of product due to slow hydration, cure, and set times and low strengths. Strength may be compensated for by the use of additional Class C fly ash but the cost of the additional fly ash may result in cost prohibitive products.
Thus, the system of the present invention minimizes the down time before cover fill may be placed and represents a significant savings of both time and money for the user. Furthermore, the present inventive composition and method allow for control variability in strength and hydration, cure, and set times of a CLSM system utilizing Class C fly which results in a Class C-based flowable fill capable of competing in markets previously inaccessible.